1. Field of the Invention
Embodiments of the present invention generally relate to methods to reduce photoresist interference in an endpoint detection process for the fabrication of photomasks useful in the manufacture of integrated circuits.
2. Description of the Related Art
The fabrication of microelectronics or integrated circuit devices typically involves a complicated process sequence requiring hundreds of individual steps performed on semiconductor, dielectric and conductive substrates. Examples of these process steps include oxidation, diffusion, ion implantation, thin film deposition, cleaning, etching and lithography. Using lithography and etching (often referred to as pattern transfer steps), a desired pattern is first transferred to a photosensitive material layer, e.g., a photoresist, and then to the underlying material layer during subsequent etching. In the lithographic step, a blanket photoresist layer is exposed to a radiation source through a reticle or photomask containing a pattern so that an image of the pattern is formed in the photoresist. By developing the photoresist in a suitable chemical solution, portions of the photoresist are removed, thus resulting in a patterned photoresist layer. With this photoresist pattern acting as a mask, the underlying material layer is exposed to a reactive environment, e.g., using wet or dry etching, which results in the pattern being transferred to the underlying material layer.
The pattern on a photomask, which is typically formed in a metal-containing layer supported on a glass or quartz substrate, is also generated by etching through a photoresist pattern. In this case, however, the photoresist pattern is created by a direct write technique, e.g., with an electron beam or other suitable radiation beam, as opposed to exposing the photoresist through a reticle. With the patterned photoresist as a mask, the pattern can be transferred to the underlying metal-containing layer using plasma etching. An example of a commercially available photomask etch equipment suitable for use in advanced device fabrication is the Tetra™ Photomask Etch System, available from Applied Materials, Inc., of Santa Clara, Calif. The terms “mask”, “photomask” or “reticle” will be used interchangeably to denote generally a substrate containing a pattern.
During processing, endpoint data from the etching of the photomasks may be used to determine whether the process is operating according to required specifications, and whether the desired results such as etch uniformity are achieved. Since each photomask generally has its own set of features or patterns, different photomasks being etched using the same process recipe may yield different endpoint data, thereby making it difficult to determine if the desired etch results are obtained for a specific photomask. Furthermore, during an etching process, the etching rate for etching the photoresist layer and the photomask may be different. Accordingly, when directing a radiation to the photoresist layer and the photomask, different thickness variation between the photoresist layer and the photomask may generate different reflective or transmissive signal to the endpoint data, therefore, making it even more difficult to determine an accurate endpoint for the photomask etching process without interfered by the photoresist thickness variation.
FIGS. 1A-1B depicts exemplary embodiments of a conventional method to detect an etch endpoint for a photomask etching process. Generally, the photomask 100 includes a metal layer 106 disposed on a phase shifting material layer 104 on a quartz substrate 102. A patterned photoresist layer 108 may be formed on the metal layer 106 utilized to transfer features onto the metal 106 and/or phase shifting material layer 104. In the case that transmissive signals are used by an endpoint detection system to collect an endpoint data, not only a first transmissive signal T1′ passing through the photoresist layer 108 is detected, but also a second transmissive signal T2′ passing through the metal layer 106 is also detected. Since the first transmissive signal T1′ may interfere with accurately analyzing the second transmissive signal T2′, precise endpoint detection is difficult, particularly in thin metal layer etching processes such as in photomask applications wherein the change in transmission may be same as the endpoint nears. Similarly, when reflective signals are utilized in a endpoint detection process, as shown in FIG. 1B, a first reflective signal R1′ interfacing the photoresist may interfere with a second reflective signal R2′ interfacing solely with the target material being etched through an opening in the photoresist, also making the endpoint detection difficult to precisely determine.
Therefore, there is an ongoing need for improved endpoint detection particularly for photomask fabrication.